If you ask people why they yawn, the most typical answer will be "because I'm tired." But sleepiness and boredom might not be the real reasons behind yawning. It actually might be a way of getting rid of hot air.

That's the finding of Princeton researcher Andrew Gallup and the University of Arizona Omar Eldakar. The pair have found that yawning frequency is correlated with what season it is, and people are more likely to yawn when their body temperature is greater than the heat around them. They argue that yawning is actually a way for our bodies to regulate brain temperature, and the apparent link to fatigue or boredom are just coincidental to that process.

Gallup explains:

"This provides additional support for the view that the mechanisms controlling the expression of yawning are involved in thermoregulatory physiology. Despite numerous theories posited in the past few decades, very little experimental research has been done to uncover the biological function of yawning, and there is still no consensus about its purpose among the dozen or so researchers studying the topic today.

"Enter the brain cooling, or thermoregulatory, hypothesis, which proposes that yawning is triggered by increases in brain temperature, and that the physiological consequences of a yawn act to promote brain cooling. I participated in a study that confirmed this dynamic after we observed changes in the brain temperature of rats before and after the animals yawned. The cooling effect of yawning is thought to result from enhanced blood flow to the brain caused by stretching of the jaw, as well as countercurrent heat exchange with the ambient air that accompanies the deep inhalation.

"According to the brain cooling hypothesis, it is the temperature of the ambient air that gives a yawn its utility. Thus yawning should be counterproductive — and therefore suppressed — in ambient temperatures at or exceeding body temperature because taking a deep inhalation of air would not promote cooling. In other words, there should be a 'thermal window' or a relatively narrow range of ambient temperatures in which to expect highest rates of yawning.

Gallup and Eldakar tested this hypothesis by measuring the yawn frequency of 160 Arizona pedestrians, eighty of whom were tested during summer - when average temperatures are 98.6 degrees Fahrenheit, just like our internal temperature - and during winter, when it's a relatively mild 71 degrees. As Gallup explains, the results showed a clear season variation:

"Our study accordingly showed a higher incidence of yawning across seasons when ambient temperatures were lower, even after statistically controlling for other features such as humidity, time spent outside and the amount of sleep the night before. Nearly half of the people in the winter session yawned, as opposed to less than a quarter of summer participants.

"Furthermore, when analyzing data for each season separately, we observed that yawning was related to the length of time a person spent outside exposed to the climate conditions. This was particularly true during the summer when the proportion of individuals yawning dropped significantly as the length of time spent outside increased prior to testing. Nearly 40 percent of participants yawned within the first five minutes outside, but the percentage of summertime yawners dropped to less than 10 percent thereafter. An inverse effect was observed in the winter, but the proportion of people who yawned increased only slightly for those who spent more than five minutes outdoors."